Photomask with three states for forming multiple layer patterns with a single exposure

ABSTRACT

The present disclosure provides one embodiment of a mask for a lithography exposure process. The mask includes a mask substrate; a first mask material layer patterned to have a first plurality of openings that define a first layer pattern; and a second mask material layer patterned to have a second plurality of openings that define a second layer pattern.

CROSS-RELATED

This patent is a continuation-in-part of U.S. Ser. No. 13/906,795entitled “Method To Define Multiple Layer Patterns Using A SingleExposure,” filed May 31, 2013, and claims the benefit of U.S.Provisional Application Ser. No. 61/823,312 entitled “Method to DefineMultiple Layer Patterns Using a Single Exposure,” filed May 14, 2013.This application is also related to U.S. patent application Ser. No.14/030,875, filed on Sep. 18, 2013, and entitled “Method to DefineMultiple Layer Patterns with a Single Exposure by E-Beam Lithography”.The entire disclosures all are hereby incorporated by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. However, these advances haveincreased the complexity of processing and manufacturing ICs and, forthese advances to be realized, similar developments in IC processing andmanufacturing are needed. In the course of integrated circuit evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased.

ICs are commonly formed by a sequence of material layers, some of whichare patterned by a photolithography process. It is important that thepatterned layers properly align or overlay with adjacent layers. Properalignment and overlay becomes more difficult in light of the decreasinggeometry sizes of modern ICs. In addition, the surface topography of anunderlying substrate, such as a semiconductor wafer, impacts thelithography imaging quality and further degrades the overlay tolerancebetween adjacent material layers. Furthermore, lithography processes area significant contributor to the overall cost of manufacturing,including processing time and the cost of masks (also referred to asphotomasks) used in the process. Therefore, what is needed is alithography method to address the above issues.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion. In addition, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact.

FIG. 1 is a top view of a photomask constructed according to aspects ofthe present disclosure in one embodiment.

FIG. 2 is a sectional view of the photomask of FIG. 1.

FIG. 3 diagrammatically illustrates a transmittance profile of thephotomask of FIG. 1.

FIGS. 4 and 5 are diagrammatical views of various exposure intensityprofiles during a lithography exposure process using the dosage map ofFIG. 1, according to one or more embodiments of the present disclosure.

FIGS. 6 and 7 are top views of latent resist patterns in respectiveresist layers using the photomask of FIG. 1.

FIGS. 8 through 20 are sectional views of a semiconductor structure atvarious fabrication stages constructed according to one or moreembodiments of the present disclosure and using the photomask of FIG. 1.

FIG. 21 is a flowchart of a method making a semiconductor structureconstructed according to one or more embodiments of the presentdisclosure.

FIG. 22 is a flowchart of a method to generate an IC pattern and make aphotomask based on the IC pattern.

FIG. 23 is a top view of a photomask constructed according to aspects ofthe present disclosure in another embodiment.

FIG. 24 diagrammatically illustrates an exposure intensity profile in alithography exposure process using the photomask of FIG. 23.

FIG. 25 is a top view of latent resist patterns in respective resistlayers using the photomask of FIG. 23.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

FIG. 1 is a top view of a photomask (reticle or mask) 10 and FIG. 2 is asectional view of the photomask 10 along the dashed line AA′ constructedaccording to one embodiment. The mask 10 is used to pattern two or moreresist layers by a single lithography exposure (or exposing, the terms“exposing” and “exposure” are often used exchangeably) process, such asultraviolet (UV) lithography, or deep UV (DUV) lithography.

The mask 10 includes a mask substrate 12 having a first transmittance S1to the exposure radiation (such as ultraviolet-UV beam, or deep UV-DUVbeam) during a lithography exposure process using the mask 10. In thepresent embodiment, the mask substrate 12 is a transparent substrate,such as a fused quartz substrate. In furtherance of the presentembodiment, the first transmittance S1 is assigned to be 100% andtransmittances S2 and S3 are defined relative to S1.

The mask 10 includes a first mask material layer 14 disposed on the masksubstrate 12. The first mask material layer 14 has a secondtransmittance S2 to the exposure radiation. The second transmittance S2is less than the first transmittance S1. In the present example, thesecond transmittance S2 ranges between about 20% and about 80%. Thefirst mask material layer 14 partially attenuates the exposureradiation. The transmittance of the first mask material layer 14 isdetermined by its composition and thickness. In the present embodiment,the first mask material layer 14 includes molybdenum silicon (MoSi).Furthermore, the first mask material layer 14 is deposited with theratio of Mo and Si tuned for desired refractive index (n) and theextinction coefficient (k) according to the wavelength of the exposureradiation. The first mask material layer 14 is designed to have asuitable thickness for expected transmittance. In one example, the firstmask material layer 14 has a thickness ranging between about 5 nm andabout 40 nm. Alternatively, the first mask material layer 14 includesother attenuating material, such as zirconium silicon oxide (ZrSiO),silicon nitride (SiN), and/or titanium nitride (TiN).

The mask 10 includes a second mask material layer 16 disposed on thefirst mask material layer 14. The second mask material layer 16 has athird transmittance S3 to the exposure radiation. The thirdtransmittance S3 is less than the second transmittance S2. In thepresent example, the second mask material layer 16 substantiallyattenuates the exposure radiation and the third transmittance S3 isaround 0% or less than 6%. In the present embodiment, the second maskmaterial layer 16 includes chromium (Cr). In one example, the secondmask material layer 16 of Cr has a thickness ranging between about 5 nmand about 80 nm. Alternatively, the second mask material layer 16 mayinclude other suitable attenuating material.

As mentioned above, the transmittances S2 and S3 are defined relative toS1. In the present embodiment, state differently, the thirdtransmittance S3 is less than 6% of the first transmittance S1 and thesecond transmittance S2 ranges between about 20% and about 80% of thefirst transmittance S1.

The mask 10 may include a capping layer formed on the second maskmaterial layer to prevent the reflection during the lithography exposureprocess using the mask 10. For one example, the capping layer includesMoSi and is patterned with the second mask material layer 16.

The first and second mask material layers 14 and 16 are patterned toform various features having respective transmittances S1, S2 and S3.Therefore, the mask 10 is also referred to as 3-state mask. Variousfeatures in different mask states (or states) response differently tothe exposure radiation during a lithography exposure process using themask 10.

The first and second mask material layers are patterned to form varioustrenches (openings) according to an integrated circuit (IC) patternhaving two layer patterns. Particularly, the first mask material layer14 is patterned according to a first layer pattern and the second maskmaterial layer 16 is patterned according to a second layer pattern. Asan example for illustration, the first mask material layer 14 ispatterned to form one or more openings 18. The second mask materiallayer 16 is patterned to form one or more openings 20.

Various openings in the mask 10 define various features in the ICpattern. In the present embodiment, the opening 18 defines a firstfeature (also referred by the numeral 18) in a first layer pattern ofthe integrated circuit, and the opening 20 defines a second feature(also referred by the numeral 20) in a second layer pattern of theintegrated circuit. The first layer pattern and the second layer patternare portion of the integrated circuit. For example, the first layerpattern is a via pattern having one or more via features and the secondlayer pattern is a metal line pattern having one or more metal lines.The via pattern and the metal line pattern are collectively a portion ofan interconnect structure in the integrated circuit.

Especially, various features in the mask 10 are assigned to respectivestates. The first feature 18 in the first layer pattern is associated toa first state having the first transmittance S1. The feature 20 in thesecond layer pattern is associated to a second state having the secondtransmittance S2. The region without pattern is referred to as field 22.The field is associated with the third state having the firsttransmittance S3.

Various features from the first and second layer patterns are combinedtogether and collectively defined in the mask 10. Particularly, thefirst layer pattern and the second layer pattern are properly combinedaccording to the corresponding spatial relationship there-between whenboth are formed on a semiconductor wafer. In the present example, thefirst feature 18 is aligned with the second feature 20 for properelectrical routing of the interconnect structure when both are formed onthe semiconductor wafer. In the mask 10, the first feature 18 isoverlapped with the second feature 20, as illustrated in FIG. 1.

In the present embodiment, the first feature 18 has a first dimension Vxin the X direction and the second feature 20 has a second dimension Lxin the X direction. Vx is less than Lx. The second feature 20 is a linefeature oriented in the Y direction perpendicular to the X direction.

The mask 10 is formed by a suitable procedure. In one embodiment, thefirst and second mask material layers are sequentially deposited by asuitable method, such as physical vapor deposition (PVD). Then thesecond mask material layer 16 is patterned by a lithography patterningprocedure that includes resist coating, lithography exposure,developing, etching, and resist removal. Similarly, the first maskmaterial layer 14 is patterned by another lithography patterningprocedure. Alternatively, the mask 10 may be formed by the methoddisclosed in the patent application (client number 2013-0782/20461.2605)hereby incorporated. Particularly, the mask material layers disposed onthe mask substrate 12 are patterned using an e-beam lithography exposureprocess according to a dosage map having three dosage levels, whichrespectively define the first feature 18, the second feature 20 and thefield 22.

FIG. 3 further diagrammatically illustrates a transmittance profile ofthe mask 10 along the dashed line A-A′. The vertical axis represents thetransmittance “T” and the horizontal axis represents a dimension alongthe X direction.

The mask 10 is used by a lithography patterning process. Variousfeatures in the mask 10 are transferred to two or more resist layers,respectively, by a single lithography exposure process.

FIG. 8 is a sectional view of a semiconductor structure 100 to bepatterned by a lithography patterning process using the mask 10. Themask 10, the semiconductor structure 100, and the method for patterningthe semiconductor structure 100 are collectively described in detailwith reference to FIGS. 1-8.

The structure 100 includes a substrate 102 and a material layer 104disposed on the substrate 102. The material layer 104 may includemultiple films with same or different materials according to differentembodiments. In one example, the material layer 104 includes a firstdielectric material layer and a second dielectric material layerdisposed on the first dielectric layer. An intermediate material layer,such as an etch stop layer may be disposed between the first and seconddielectric material layers.

A first resist layer 108 is coated on the material layer 104 and asecond resist layer 112 is disposed on the first resist layer 108. Thefirst and second resist layers 108 and 112 may be different incomposition. For example, with different resist composition, the tworesist layers have different sensitivities (exposure threshold). In oneembodiment, other material layers, such as 106 and 110, may be formedbetween the resist layers and/or below the resist layers for one or morepurpose, such as attenuation and/or isolation.

Referring to FIGS. 4 through 7, one way to describe the exposure of tworesist layers 108 and 112 is to consider exposure intensities for thetwo resist layers.

FIG. 4 illustrates an exposure intensity profile 36 of the second resistlayer 112, which is the upper of the two resist layers in the presentembodiment. Specifically, FIG. 4 graphically illustrates the exposureintensity distribution (vertical scale) across the width of the secondresist layer 112 to be exposed (horizontal scale) corresponding to thetransmittance profile along the dashed line A-A′ of the mask 10, whichis illustrated in FIG. 3.

FIG. 5 illustrates an exposure intensity profile 38 of the first resistlayer 108, which is the lower of the two resist layers in the presentembodiment. Specifically, FIG. 5 graphically illustrates the exposureintensity distribution (vertical scale) across the width of the firstresist layer 108 to be exposed (horizontal scale) corresponding to thetransmittance profile along the dashed line A-A′ in the mask 10. Theexposure intensity profile 38 may be different from the exposureintensity profile 36 due to various factors, which include theattenuation and scattering of the exposure radiation from the secondresist layer 112 and additionally from the material layer 110 (ifpresent).

By a single lithography exposure process using the mask 10 with the ICpattern defined thereon, latent patterns 40 and 42 are formed on thefirst and second resist layers 108 and 112, respectively, as illustratedin FIGS. 7 and 6. The latent pattern of a resist layer refers to theexposed pattern on the resist layer, which eventually becomes a physicalresist pattern, such as by a developing process. In the present case,the latent patterns illustrated in FIGS. 6 and 7 are correspondingimages of the exposed portions with exposing intensity equal to orgreater than the respective exposure threshold.

In the present embodiment, the latent pattern 40 on the first resistlayer 108, as illustrated in FIG. 7, includes a first feature 48. Thelatent pattern 42 on the second resist layer 112, as illustrated in FIG.6, includes a second feature 46. The latent pattern 42 on the secondresist layer 112 and the latent pattern 40 on the first resist layer 108are different from each other. Therefore, by one exposure process, tworesist layers are exposed with respective patterns. This is furtherexplained below.

Each resist material has its respective exposure threshold to radiation.When the exposing intensity is equal to or greater than the exposurethreshold, the corresponding portion of the resist is chemically changedsuch that it will be developed (e.g., it is removed by the developerwhen the resist is positive tone) in a developing process. When theexposing intensity is less than the exposure threshold, thecorresponding portion of the resist is not chemically changed to bedeveloped (e.g., it remains during the developing process when theresist is positive tone). It is understood that the term “changed” meansthat the resist has sufficiently changed to respond differently, e.g.,as exposed positive-tone resist responds in the development process. Inone example where the resist is positive tone, only portions of theresist exposed with exposing intensity equal to or greater than theexposure threshold are removed by a suitable developer during thedeveloping process. Other portions of the resist unexposed or exposedwith exposing intensity less than the exposure threshold remain afterthe developing process.

In another example where the resist is negative tone, the portions ofthe resist unexposed or exposed with exposing intensity less than theexposure threshold are removed by a suitable developer during thedeveloping process. Other portions of the resist exposed with exposingintensity equal to or greater than the exposure threshold remain afterthe developing process.

In the present embodiment, the first and second resist layers are bothpositive tone. During the lithography exposure process using the mask10, both the first and second resist layers are exposed to form latentpatterns 40 and 42 as illustrated in FIGS. 7 and 6, respectively, due toone or more factors.

In one embodiment, the first resist layer and the second resist layerare designed to have different exposure thresholds. The first resistlayer 108 has a relatively high exposure threshold T_(h)1 and the secondresist layer 112 has a relatively low exposure threshold T_(h)2, i.e.,less than that of the first resist layer.

In FIG. 4, the exposure intensity profile 36 of the second resist layer112 includes a portion corresponding to the second feature 20 and thefirst feature 18. Accordingly, the exposure intensity profile 36includes a step shoulder 52 with an intensity I₂ associated with thesecond transmittance state (S2). The exposure intensity profile 36further includes a peak 54 having an intensity I₁ associated with thefirst transmittance state (S1). The resist material of the second resistlayer 112, the mask 10, and the exposure radiation intensity aredesigned such that the second threshold T_(h)2 is less than theintensity I₂. Thus, the second feature 20 in the mask 10 is imaged toform the second feature 46 in the latent pattern 42 during thelithography exposure process, as illustrated in FIG. 6. The firstfeature 18 is also imaged to the latent pattern 42 but is overlappedwith the second feature 46.

In FIG. 5, the exposure intensity profile 38 of the first resist layer108 includes a portion corresponding to the first feature 18 and thesecond feature 20 of the mask 10. Accordingly, the exposure intensityprofile 38 includes a step shoulder 58 having an intensity I₄ associatedwith the second transmittance state (S2). The exposure intensity profile38 further includes a peak 60 having an intensity I₃ associated with thefirst transmittance state (S1). The intensities I₃ and I₄ may be lessthan the intensities I₁ and I₂, respectively, due to one or moreattenuation mechanisms. The resist material of the first resist layer108, the mask 10, and the exposure radiation intensity are designed suchthat the first threshold T_(h)1 is less than the intensity I₃ but isgreater than the intensity I₄. Thus, the second feature 20 of the mask10 is not imaged in the latent pattern but the first feature 18 of themask 10 is imaged to form the first latent feature 48 in the latentpattern 40 during the lithography exposure process, as illustrated inFIG. 7.

Since the first resist layer 108 has a higher exposure threshold T_(h)1,the first latent pattern 40 formed thereon by the lithography exposureprocess is different from that of the second resist layer 112. By onelithography exposure process using the mask 10, two different latentpatterns 40 and 42 are respectively formed in the two resist layers 108and 112.

By properly choosing dimensions of various features in the IC pattern,as defined on the mask 10, the first and second latent patterns areformed on the respective resist layer with respective features of properdimensions (dimensions on wafer or DOW) in best focus (BF). In oneexample, the first feature 18 of the first layer pattern is tunedaccording to a first size bias to form the latent pattern 40 in thefirst resist layer 108 with proper dimensions. The second feature 20 ofthe second layer pattern is tuned with a second size bias different fromthe first size bias to form the corresponding latent pattern 42 in thesecond resist layer 112 with proper dimensions.

In one example illustrated in FIG. 1, the first feature 18 is designedwith a first dimension Vx in the X direction relative to thecorresponding dimension Lx of the second feature 20, where Vx is lessthan Lx.

For the lithography exposure process, the mask 10 is designed to havedifferent biases to the features in the first layer pattern and thesecond layer pattern. The bias includes two or more freedoms, includingsize and transmittance, to tune the CDs of various features.

In another embodiment, an attenuation mechanism is provided such thatthe exposing intensity to the first resist layer is less than theexposing intensity to the second resist layer to form different latentpatterns on respective resist layers. In this embodiment, the exposurethreshold to the first resist layer 108 may be chosen as same as that ofthe second resist layer 112 or alternatively different. In one example,the second resist layer 112 attenuates the exposure radiation such thatonly a portion of the exposing beam reaches to the first resist layer.In another example illustrated in FIG. 8, the attenuating material layer110 is inserted between the first and second resist layers. Theattenuating material layer 110 absorbs the exposing radiation such thatthe exposing beam reaching the first resist layer 108 is only a portionof the exposing radiation projected on the second resist layer 112. Thusthe exposing intensity to the first resist layer 108 is less than theexposing intensity to the second resist layer 112. Accordingly, based onthe exposing intensity and the exposure threshold, the latent pattern onthe first resist layer 108 is different from the latent pattern formedon the second resist layer 112. Particularly, when the first exposurethreshold T1 associated with the first resist layer 108 is greater than14 and less than 13 (as illustrated in FIG. 5), the second feature 20defined in the mask 10 is not imaged to the first resist layer 108. Thefirst feature 18 is imaged to the first resist layer 108 by thelithography exposure process, thereby forming the latent feature 40 asillustrated in FIG. 7. As a comparison, the second exposure threshold T2associated with the second resist layer 112 is less than both I1 and I2(as illustrated in FIG. 4), both the first feature 18 and the secondfeature 20 defined in the mask 10 are imaged to the second resist layer112, thereby forming the latent feature 42 as illustrated in FIG. 6.

In various embodiments, by properly choosing transmittance of the mask10; choosing the exposure threshold through tuning of the resistmaterials; choosing the exposing intensity through various attenuationmechanisms (resist or inserting an attenuating material layer);adjusting various dimensions of various features in the IC pattern, or acombination thereof, the different latent patterns are formed onrespective resist layers with proper dimensions.

Thereafter, the two resist layers are developed to form a first resistpattern in the first resist layer and a second resist pattern in thesecond resist layer. Other manufacturing operations follow to transferthe two resist patterns to the substrate. In one example, one or moreetch operations are implemented to transfer the two resist patterns torespective underlying material layers on the substrate.

By the disclosed method, two resist layers are simultaneously exposed toform respective patterns by one lithography exposure process. Therefore,both the manufacturing cost and manufacturing cycle time are reduced.Other benefits may present in various embodiments. In one embodiment,the two resist patterns, therefore, the two respective patternstransferred to the underlying material layers, are intrinsically alignedsince they are printed from the same IC pattern.

FIGS. 8-20 are sectional views of the semiconductor structure 100 atvarious fabrication stages. The method to simultaneously pattern tworesist layers using the mask 10 and the semiconductor structure madethereby are further described below according to one embodiment withreference to FIGS. 1-20.

Referring to FIG. 8, a semiconductor substrate 102 is provided. In thepresent embodiment, the semiconductor substrate 102 includes silicon.Alternatively, the substrate 102 includes germanium, silicon germaniumor other suitable semiconductor material, such as diamond, siliconcarbide or gallium arsenic. The substrate 102 may further includeadditional features and/or material layers, such as various isolationfeatures formed in the substrate. The substrate 102 may include variousp-type doped regions and/or n-type doped regions configured and coupledto form various devices and functional features. All doping features maybe achieved using a suitable process, such as ion implantation invarious steps and techniques. The substrate 102 may include otherfeatures, such as shallow trench isolation (STI) features. The substrate102 may also include a portion of an interconnect structure thatincludes metal lines in various metal layers, via features to providevertical connection between the metal lines in the adjacent metallayers, and contact features to provide vertical connection between themetal lines in the first metal layer and various device features (suchas gates, sources and drains) on the substrate.

Still referring to FIG. 8, various material layers are formed on thesubstrate 102. In the present embodiment, a dielectric material layer104 is formed on the substrate 102. The dielectric material layer 104may include plurality of dielectric films. In the present embodiment,the dielectric material layer 104 includes a first interlayer dielectric(ILD) material 104A formed on the substrate 102. The first ILD materiallayer 104A includes a dielectric material, such as silicon oxide, low kdielectric material, other suitable dielectric material or combinationthereof.

The dielectric material layer 104 includes a second ILD material layer104B formed over the first ILD material layer 104A. The second ILDmaterial layer 104B is similar to the first ILD material layer 104A interms of composition and formation. For example, the second ILD materiallayer 104B includes a dielectric material, such as silicon oxide, low kdielectric material, other suitable dielectric material or combinationthereof.

The dielectric material layer 104 includes an etch stop layer 104Cformed between the first and second ILD material layers. The etch stoplayer 104C has an etch selectivity to the ILD material and functions tostop etch during subsequent operation to pattern the ILD materiallayers. The etch stop layer 104C is different from the ILD material incomposition and includes another dielectric material, such as siliconnitride, silicon oxynitride or silicon carbide. Various dielectricmaterials may be deposited by a suitable technique, such as chemicalvapor deposition (CVD), spin-on coating or other suitable method.

Two resist layers are subsequently formed on the dielectric materiallayer 104. Specifically, a first resist layer 108 is formed over thedielectric material layer 104. The first resist layer 108 is formed byspin-on coating or other suitable technique. A second resist layer 112is formed over the first resist layer 108. The second resist layer 112is formed by spin-on coating or other suitable technique. Other steps,such as baking, may follow the coating of each resist layer. The firstand second resist layers may have similar or different compositions fromeach other, according to various embodiments. Two resist layers includea same resist material or different resist materials sensitive toexposure radiation.

In one embodiment, the second resist layer 112 is different from thefirst resist layer 108 and is formed directly on the first resist layer108. The first and second resist layers are configured to be exclusivelydissolved in separate, respective developers. Specifically, a firstdeveloper is used to develop the first resist layer 108 and a seconddeveloper is used to develop the second resist layer 112. The firstdeveloper is different from the second developer. The first resist layeris dissoluble in the first developer but indissoluble in the seconddeveloper. The second resist layer is dissoluble in the second developerbut indissoluble in the first developer. In another embodiment, althoughthe two resists are mutually indissoluble, they could dissolve in thesame developer. In one example, the first and second resist layers arechosen to have different exposure thresholds. In another example, thesecond resist layer 112 attenuates the exposing radiation during thelithography exposure process such that the exposing radiation projectedon the second resist layer 112 is partially absorbed and only a portionof the exposing radiation reaches the first resist layer 108. Thus theexposing intensities to the first and second resist layers aredifferent. Specifically, the exposing intensity to the first resistlayer 108 is less than the exposing intensity to the second resist layer112. In this case, the exposure thresholds of the first and secondresist layers may be chosen to be the same, or different. In anotherexample, the first resist layer 108 has a thickness ranging betweenabout 20 nm and about 60 nm. In another example, the second resist layer112 has a thickness ranging between about 20 nm and about 40 nm.

In another embodiment, a material layer 110 is formed between the firstand second resist layers. In this embodiment, the two resist layers maybe same in composition or different. The material layer 110 is insertedthere-between to serve one or more functions. In one example, thematerial layer 110 separates the first and second resist layers fromeach other if those two resist layers are mutually dissoluble. Inanother example, the material layer 110 functions to absorb the exposingradiation such that the exposing radiation projected on the secondresist layer 112 is partially absorbed and only a portion of theexposing radiation reaches the first resist layer 108. Thus the exposingintensity to the first resist layer 108 is less than the exposingintensity to the second resist layer 112. In another example, thematerial layer 110 functions as a hard mask during subsequent operationsto pattern the dielectric material layer 104. The material layer 110 isformed on the first resist layer 108 before the coating of the secondresist layer 112.

The material layer 110 includes a dielectric material, such as aluminumoxide (Al₂O₃), silicon oxide (SiO₂), silicon nitride (SiN), titaniumoxide (TiO), or other suitable material. The material layer 110 isformed by spin-on coating or low temperature deposition without damageto the underlying resist layer 108. For example, the material layer 110of aluminum oxide is deposited by spin-on coating. In another example,the material layer 110 of silicon oxide, silicon nitride, or titaniumoxide, is formed by a low temperature deposition, such as CVD at lowtemperature. In one example, the material layer 110 has a thicknessranging between about 10 nm and about 20 nm.

In another embodiment, a second material layer 106 is formed between thedielectric material layer 104 and the first resist layer 108. In thepresent embodiment, the second material layer 106 functions as a hardmask layer during the subsequent operations to pattern the dielectricmaterial layer 104. The material layer 106 may be different from thematerial layer 110 or alternatively same. For example, the materiallayer 106 may include aluminum oxide. The second material layer 106 isformed on the dielectric material layer 104 before the coating of thefirst resist layer 108. The second material layer 106 may include one ormore films to enhance the operations of patterning the dielectricmaterial layer 104.

Referring to FIG. 9, a lithography exposure process is implemented usingthe mask 10 to simultaneously expose both the first and second resistlayers, thereby forming latent patterns on respective resist layers.During the lithography exposure process, the IC pattern defined in themask 10 is imaged to the second resist layer 112 and the first resistlayer 108. A first latent pattern 40 is formed in the first resist layer108 and a second latent pattern 42 is formed in the second resist layer112. A latent pattern is referred to as a portion of the resist layerthat is exposed but not developed yet. The first and second latentpatterns are different from each other due to different exposingintensities, different exposure thresholds or both, as described abovewith reference to FIGS. 1 through 7. However, the first and secondlatent patterns are related since both are images of the same IC patterndefined on the mask 10. In the present example, the first latent pattern40 includes a first latent feature 48 associated with the via feature 18and the second latent pattern 42 includes a second latent feature 46associated with the line feature 20 defined in the mask 10. Otheroperations, such as post-exposure-baking (PEB), may follow thelithography exposure process.

Referring to FIG. 10, the second resist layer 112 is developed by thecorresponding developer. In the present embodiment, both the first andsecond resist layers are positive tone. The exposed portion (the latentfeature 46) of the second resist layer 112 is removed in the developer,thereby forming a patterned second resist layer having an opening 118associated with the second latent feature 46. Other operations, such ashard baking, may follow the developing process.

Referring to FIG. 11, an etch process is applied to selectively etch thematerial layer 110 and remove the portion of the material layer 110aligned within the opening 118. The etch process and the etchant areproperly chosen for selective etch without damage to the resist.

Referring to FIG. 12, the first resist layer 108 is developed by thecorresponding developer. In the present embodiment, the first resistlayer is positive tone. The exposed portion (the first latent feature48) is removed in the developer, thereby forming a patterned firstresist layer having an opening 120 associated with the first latentfeature 48. Other operations, such as hard baking, may follow thedeveloping process.

Referring to FIG. 13, another etch process is applied to selectivelyetch the second material layer 106 to remove the portion within theopening 120.

Referring to FIG. 14, the second resist layer 112 may be removed by asuitable process, such as wet stripping or plasma ashing.

Other operations are applied to transfer the openings 118 and 120 to therespective material layers. One embodiment is further described below.

Referring to FIG. 15, an etch process is applied to selectively etch thesecond ILD material layer 104B within the opening 120, thereby forming atrench 122 in the second ILD material layer 104B. The etch process stopson the etch stop layer 104C. The etch process is properly chosen to formthe trench 122. For example, dry etch, wet etch, or a combinationthereof, may be applied for transferring the opening 120 to the secondILD material layer 104B, forming the trench 122.

Referring to FIG. 16, another etch process is applied to selectivelyetch the etch stop layer 104C within the trench 122, using a suitableetch technique and etchant. In one embodiment, a wet etch may be appliedto open the etch stop layer 104C. For example where the etch stop layer104C includes silicon oxide, a hydrofluoride (HF) may be used as etchantto etch the etch stop layer 104C.

Referring to FIG. 17, a trimming process is applied to trim the firstresist layer 108, thereby transferring the opening 118 from the materiallayer 110 to the first resist layer 108. The uncovered portion of thefirst resist layer 108 is removed by the trimming process. In oneembodiment, the trimming process is similar to a resist strip process.For example, the trimming process implements wet stripping.

Referring to FIG. 18, an etch process is applied to etch the materiallayer 106 within the opening 118, thereby transferring the opening 118to the material layer 106. In one embodiment, the material layer 106 andthe material layer 110 includes a same material (such as aluminumoxide), the etch process opens the material layer 106 and removes thematerial layer 110 as well.

Referring to FIG. 19, another etch process is applied to selectivelyetch both the first ILD material layer 104A and the second ILD materiallayer 104B using the material layer 106 as an etch mask, thereby forminga first trench 124 for the via feature in the first ILD material layer104A and a second trench 126 for the metal line in the second ILDmaterial layer 104B. In the present embodiment, the first and second ILDmaterial layers include a same dielectric material. The etch processrecesses both the first and second ILD material layers. The etch processis properly chosen for selective etch. For example, dry etch may beapplied to form the via trench 124 and metal line trench 126 inrespective ILD material layers.

In some embodiments, another etch stop layer is disposed between thesubstrate 102 and the first ILD material layer 104A such that the etchprocess properly stops on the etch stop layer. In this case, the etchstop layer can be subsequently opened by another etch for properelectrical connection. In another embodiment, an underlying metal layeris formed below the first ILD material layer and the via trench 126 isproperly aligned with the underlying metal line for electricalconnection. Other operations may be subsequently implemented. Forexample, the first resist layer 108 may be removed by wet stripping orplasma ashing.

Although the procedure to form the via trench 124 and the metal linetrench 126 is provided above according to one or more embodiments, otherprocedure may be alternatively applicable to form the via trench 124 andthe metal line trench 126 using the patterned first and second resistlayers.

In another embodiment where the material layer 110 is not present,various etch operations applied to the material layer 110 areeliminated.

Referring to FIG. 20, via feature 128 and metal line 130 are formed by asuitable procedure. In one embodiment, a conductive material, such asmetal or metal alloy, is filled in the via trench 124 and the metal linetrench 126 (FIG. 19) by deposition, such as physical vapor deposition(PVD), plating or a combination thereof. A chemical mechanical polishing(CMP) process is applied to remove excessive conductive material and toplanarize the top surface.

In another embodiment, the material layer 106 may serve as a polishingstop layer and may be removed after the CMP process by an etch process.In a particular example, copper is used as the conductive material. Infurtherance of this example, a copper seed layer is formed by PVD.Thereafter, bulk copper is filled in the trenches 124 and 126 byplating. A CMP process is subsequently applied to remove the excessivecopper and planarize the top surface. In yet another embodiment, alining material, such as titanium nitride, is formed on the sidewalls ofthe via trench 124 and the metal line trench 126 before filling in thetrenches with the conductive material. The lining layer is deposited bya proper technique, such as PVD or CVD. The lining layer may function asa diffusion barrier and adhesive layer for integrity of the interconnectstructure.

Although not shown, other processing operation may be presented to formvarious doped regions such as source and drain regions and/or devicesfeatures such as gate electrode. In one example, the substrate mayalternatively include other material layer to be patterned by thedisclosed method, such as another patterned metal layer. In anotherexample, additional patterning steps may be applied to the substrate toform a gate stack. In another example, the source and drain features areof either an n-type dopant or a p-type dopant are formed by aconventional doping process such as ion implantation.

FIG. 21 is a flowchart of a method 200 of exposing two resist layerswith respective latent patterns by single lithography exposure process,constructed according various aspects in one or more embodiments. Themethod 200 starts at 202 with a substrate, such as a semiconductorwafer. The substrate may further include one or more material layers,such as one or more patterned layers and one or more layers to bepatterned.

At operation 204, a first resist layer is formed on the substrate.Forming of the first resist layer includes coating the first resistlayer on the substrate by a suitable technique, such as spin-on coating.Other manufacturing steps, such as baking, may further be applied to thefirst resist layer.

At operation 206, a second resist layer is formed on the first resistlayer. Forming of the second resist layer includes coating the secondresist layer on the substrate by a suitable technique, such as spin-oncoating. Other manufacturing steps, such as baking, may further beapplied to the second resist layer.

The first and second resist layers may be the same or different incomposition. In one embodiment, the second resist layer is differentfrom the first resist layer in the exposure threshold. In anotherembodiment, the second resist layer is different from the first resistlayer as they are mutually indissoluble. In another embodiment, amaterial layer is inserted between the first and second resist layer forseparation, attenuation and/or etch mask.

The method 200 proceeds to operation 208 by performing a lithographyexposure process using the mask 10 having three states to simultaneouslyexpose both the first and second resist layers, thereby forming a firstlatent pattern in the first resist layer and a second latent pattern inthe second resist layer. The first and second patterns are differentfrom each other and define respective patterns to be transformed indifferent material layers.

The lithography exposure process exposes the two resist layers accordingusing the mask 10 having three states. The three states S1, S2 and S3are constructed to define various features from two layer patterns.Especially, the IC pattern defined in the mask 10 includes a firstplurality of features for a first layer pattern and a second pluralityof features for a second layer pattern. The first plurality of featuresare defined in the first state S1 and the second plurality of featuresare defined in the second S2 in the mask 10.

In the mask 10, the transmittances of various states and dimensions ofthe first layer patent and second layer pattern are designed accordingto exposure thresholds, and attenuation to the exposing intensity suchthat the features in the first layer pattern and the features in thesecond layer patter are imaged to the first and second resist layers,respectively, to form respective latent patterns with proper dimensions.Other steps may be implemented. In one embodiment, a post exposurebaking process may be applied to the first and second resist layersafter the lithography exposure process.

The method 200 proceeds to operation 210 by developing the second resistlayer to form the patterned second resist layer. The second resist layerwith the second latent pattern is converted to the patterned secondresist layer with various openings thereby. In one embodiment, thesecond resist layer is positive tone, and the portions of the secondresist layer associated with the second latent pattern are removed bythe corresponding developer, resulting in the openings in the secondresist layer (the second resist layer with the second pattern convertedfrom the second latent pattern).

The method 200 proceeds to operation 212 by developing the first resistlayer to form the patterned first resist layer. The first resist layerwith the first latent pattern is converted to the patterned first resistlayer with various openings. In one embodiment, the first resist layeris positive tone, and the portions of the first resist layer associatedwith the first latent pattern are removed by the correspondingdeveloper, resulting in the openings in the first resist layer.Thereafter, other steps may be implemented. In one embodiment, one ormore baking processes may be applied to the first and second resistlayers collectively or separately.

The method 200 proceeds to operation 214 by transferring the firstpattern and the second pattern to the substrate or underlying materiallayers on the substrate. The operation 214 may include one or more etchprocesses, such as those various embodiments associated with FIGS. 8through 20. In one embodiment, a via trench and a metal line trench areformed in respective ILD material layers. Other manufacturing operationsmay be implemented before, during or after the method 200. In oneembodiment, a procedure including metal deposition and CMP isimplemented thereafter to form a via feature (or contact feature) and ametal line overlapped and aligned.

The present disclosure also provides a method for generating a tape-outdata for mask fabrication, such as the mask 10. FIG. 22 is a flowchartof a method 250 of generating the tape-out data that defines an ICpattern thereon and is used to fabricate a mask (the mask 10 in FIG. 1in the present embodiment) having three states.

The method 250 begins at operation 252 by receiving an IC design layoutthat includes a first layer pattern and a second layer pattern. Thefirst layer pattern is designed to expose a first resist layer by alithography exposure process and furthermore, to be formed in a firstmaterial layer on a substrate (such as a semiconductor wafer) and thesecond layer pattern is designed to expose a second resist layer by thelithography exposure process and furthermore, to be formed in a secondmaterial layer overlying the first material layer. In one embodiment forillustration, the first layer pattern includes a via pattern having avia feature (or a plurality of via features), and the second layerpattern is a metal line pattern having one metal line (or a plurality ofmetal lines).

The method 250 proceeds to operation 254 by tuning a first feature ofthe first layer pattern according to a first bias. The first bias ischosen such that the first resist layer is exposed to form a firstlatent pattern having the first feature (a via feature in the presentexample) with proper dimensions. The first bias may include firstintensity bias (through tuning the first transmittance) and first sizebias (through tuning the dimension of the first feature). The tuning ofthe first feature includes adjusting the first transmittance S1 in thefirst state of the mask 10 (S1 is associated with the first featuresince it is used to define the first feature in the mask 10) andadjusting the dimensions of the first feature. The transmittance anddimensions of the first feature collectively determine the criticaldimensions of the first feature when formed on a semiconductor waferduring the lithography exposure process. When the first layer patternincludes more features, each is tuned in the same way until all featuresin the first layer pattern is exhausted.

The method 250 proceeds to operation 256 by tuning a second feature ofthe second layer pattern according to a second bias. The second bias ischosen such that the second resist layer is exposed to form a secondlatent pattern having the second feature (a metal line feature in thepresent example) with proper dimensions. The second bias may includesecond intensity bias (through tuning the second transmittance) andsecond size bias (through tuning the dimension of the second feature).The tuning of the second feature includes adjusting the secondtransmittance S2 in the second state of the mask 10 (S2 is associatedwith the second feature since it is used to define the second feature inthe mask 10) and adjusting the dimensions of the second feature. Thetransmittance and dimensions of the second feature collectivelydetermine the critical dimensions of the second feature when formed onthe semiconductor wafer during the lithography exposure process. Whenthe second layer pattern includes more features, each is tuned in thesame way until all features in the second layer pattern is exhausted.

The first and second biases are different from each other in order todifferentiate exposure intensities and form different latent patterns onthe two resist layers.

In one embodiment, the mask material layers for the mask 10 aredetermined (such as MoSi and Cr) according to various factors, thetunings of the first feature and the second feature include adjustingthe thicknesses of the mask material layers and the dimensions of thefirst and second features, respectively. In another embodiment, the maskmaterial layers for the mask 10 are determined (such as MoSi and Cr)with respective composition and thickness, the tunings of the firstfeature and the second feature include adjusting the dimensions of thefirst and second features, respectively.

By different biases for the first layer pattern and the second layerpattern, the exposure radiation intensity difference between the firstand second layer patterns is achieved. As an example illustrated in FIG.5, the intensity I3 associated with the first layer pattern is differentfrom (Specifically, greater than) the intensity I4 associated with thesecond layer pattern due to different biases. With this intensitydifference, the first layer pattern can be selectively imaged to thefirst resist layer while the second layer pattern is not imaged to thefirst resist layer (such as by choosing different exposure thresholdsand/or attenuation) during the lithography exposure process.

The method 250 proceeds to operation 258 by combining the first andsecond adjusted (with different transmittance and possibly further withdimensions adjustment) layer patterns to form a combined IC pattern. Thecombined IC pattern is a sum of the first and second adjusted layerpatterns associated with respective transmittances. As illustrated inFIG. 1, the first adjusted layer pattern includes the first feature (thevia feature) 18 with first transmittance S1 and possibly with the firstsize bias. The second adjusted pattern includes the metal line 20 withthe second transmittance S2. The first and second adjusted patterns arecombined according to the spatial relationship when formed on thesubstrate (the spatial relationship between the via pattern and metalline pattern). In the embodiment illustrated in FIG. 1, the via feature18 and the metal line 20 are aligned and overlapped when formed in thesubstrate in the top view. In the present example, the via feature 18has a dimension Vx and the metal line 20 has a dimension Lx greater thanVx in the combined IC pattern since the first bias and the second biasare different.

The method 250 proceeds to operation 260 by generating a tape-out datafor mask fabrication according to the combined IC pattern. The tape-putdata is constructed in a proper format that defines various features andvarious states associated with the respective features. Particularly,the combined IC pattern defined in the tape-out data includes threestates and defines various features with respective state. Particularly,the first layer pattern is defined with the first state S1, the secondlayer pattern is defined with the second state S2 and the field isdefined with the state S3. In furtherance of the present embodiment, thefirst feature 18 in the first layer pattern is defined in the firststate S1, the second feature 20 in the second layer pattern is definedin the second state S2, and the field is defined in the third state S3.The tape-out data is defined in a proper data format, such as in GDSformat.

The method 250 may proceed to operation 262 by making a mask (the mask10 in the present example) according to the tape-out data that definesthe combined IC pattern with different states. The method of making themask 10 according to the tape-out data is described above in FIGS. 1 and2. The first mask material layer 14 and the second mask material layer16 are deposited on the mask substrate 12 and patterned according to thefirst layer pattern and the second layer pattern defined in the tape-outdata, respectively.

In the present embodiment, the mask 10 is formed by the method disclosedin the patent application (client number 2013-0782/20461.2605)incorporated hereby. In this method, two resist layers are coated on themask material layers and are exposed by a single exposure process usinge-beam. Particularly, the IC pattern defined in the tape-out data isused as a dosage map for the e-beam lithography exposure process topattern the two resist layers. The various states (S1, S2 and S3) of theIC pattern defined in the tape-out data represent various dosage levels(D1, D2, and D3, respectively) during the e-beam lithography exposureprocess. In furtherance of the embodiment, the first dosage D1 isgreater than the second dosage D2 and the second dosage D2 is greaterthan the third dosage D3. In the present example, the third dosage D3 isaround zero.

In an alternative embodiment, after the operation 256, a first tape-outdata is generated according to the first layer pattern and a secondtape-out data is generated according to the second layer pattern. Thefirst and second tape-out data are used to pattern the first and secondmask material layers by respective e-beam lithography exposureprocesses. Various etch processes are further applied to the first andsecond material layers to transfer the first and second layer patternsfrom the first and second resist layers to the first and second maskmaterial layers, respectively, forming the mask 10.

Another embodiment of a mask having three states is provided below. FIG.23 is a top view of a mask 300 having three states and defines a firstlayer pattern having a first feature 18, a second layer pattern havingsecond features 20, and a field 22. The first layer pattern, the secondlayer pattern and the field have different states, such as differenttransmittances at present embodiment. Especially, the first layerpattern is in a first state having a first transmittance S1, the secondlayer pattern is in a second state having a second transmittance S2 lessthan S1, and the field is in a third state having a third transmittanceS3 less than S2. Similarly, the mask 300 includes a mask substrate, afirst mask material layer and a second mask material having respectivetransmittance S1, S2 and S3. In one example, the first mask materiallayer includes MoSi and the second mask material layer includes Cr.Furthermore, the first mask material layer is patterned to define thefirst layer pattern and the second mask material layer is patterned todefine the second layer pattern. In the present case, the first feature18 spans a first dimension in the X direction and the second features 20span a second dimension less than the first dimension in the Xdirection.

FIG. 24 diagrammatically illustrates an exposure intensity profile 310in a lithography exposure process using the mask 300 to simultaneouslypattern a first resist layer and a second resist layer with respectivepatterns. In the present example, the exposure intensity profile 310 isan exposure intensity profile of the second resist layer. The exposureintensity profile of the first resist layer is similar to the exposureintensity profile 310.

FIG. 25 collectively illustrates a top view of latent resist patterns inrespective resist layers by a lithography exposure process using themask 300. More specifically, a first latent pattern formed in the firstresist layer includes a first latent feature 48 associated with thefirst feature 18, and a second latent pattern formed in the secondresist layer includes second latent features 46 associated with thesecond features 20.

In this embodiment, the first and second biases to the first and secondlayer patterns are different from those in FIGS. 1 and 2. Accordingly,the second latent feature 46 aligned with the first latent feature 48have a large dimension along the X direction in the overlapped portionfor more alignment window.

Various advantages may present in different embodiments of the presentdisclosure. In one embodiment, the mask 10 having three states can beused to simultaneously pattern two resist layers with respective latentpatterns by a single lithography exposure. In another embodiment, thetunings of the first and second layer patterns have more freedomsincluding transmittance tuning and size tuning. Although embodiments ofthe present disclosure have been described in detail, those skilled inthe art should understand that they may make various changes,substitutions and alterations herein without departing from the spiritand scope of the present disclosure. For example, the mask 10 mayinclude more three states each having respective transmittance. Infurtherance of the embodiment, three mask material layers withrespective transmittance are deposited and patterned to define featuresfrom three layer patterns, respectively. In another embodiment, the mask10 may be designed as a reflective mask for extreme UV (EUV)lithography. In this case, the mask substrate 12 includes a low thermalexpansion material (LTEM) substrate and the first mask material layer 14includes reflective multiple layers, such pairs of Mo and Si layersdesigned to reflect EUV radiation.

Thus, the present disclosure provides one embodiment of a mask for alithography exposure process. The mask includes a mask substrate; afirst mask material layer patterned to have a first plurality ofopenings that define a first layer pattern; and a second mask materiallayer patterned to have a second plurality of openings that define asecond layer pattern.

The present disclosure also provides an embodiment of a method thatincludes forming a first resist layer on a semiconductor substrate;forming a second resist layer over the first resist layer; andperforming a lithography exposure process to the first resist layer andthe second resist layer using a three-state mask, thereby forming afirst latent pattern in the first resist layer and a second latentpattern in the second resist layer.

The present disclosure also provides one embodiment of a method thatincludes receiving an integrated circuit (IC) design structure having afirst layer pattern and a second layer pattern, wherein the first layerpattern defines at least a first feature to be formed in a firstmaterial layer on a substrate and the second layer pattern defines atleast a second feature to be formed in a second material layer disposedon the first material layer; tuning the first feature according to afirst bias; tuning the second feature according to a second biasdifferent from the first bias; thereafter combining the first and secondfeatures to form a combined IC pattern; and generating a tape-out datathat defines the combined IC pattern for mask making.

The present disclosure provides another embodiment of a mask used in alithography exposure process. The mask includes a mask substrate; afirst mask material layer disposed on the mask substrate; and a secondmask material layer disposed on the first mask material layer, whereinthe first and second mask material layers are patterned to define threestates different from each, defining a first layer pattern, a secondlayer pattern and a field region, respectively.

In one embodiment of the mask, the mask substrate has a firsttransmittance to an exposure radiation of the lithography exposureprocess; the first mask material layer has a second transmittance lessthan the first transmittance; and the second mask material layer has athird transmittance less than the second transmittance. In antherembodiment, the three states include a first state having the firsttransmittance, a second state having the second transmittance and athird state having the third transmittance. In one example, the firsttransmittance is 100%; the third transmittance is 0; and the secondtransmittance ranges between about 20% and about 80%. In yet anotherembodiment, the first layer pattern includes a first feature defined ina first opening of the first mask material layer; and the second layerpattern includes a second feature defined in a second opening of thesecond mask material layer. In yet another embodiment, the first maskmaterial layer includes molybdenum silicon (MoSi); and the second maskmaterial layer includes chromium (Cr).

The foregoing has outlined features of several embodiments. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A mask for a lithography exposure process,comprising: a mask substrate; a first mask material layer over the masksubstrate and patterned to have a first plurality of openings thatdefine a first layer pattern; and a second mask material layer over thefirst mask material layer and patterned to have a second plurality ofopenings that define a second layer pattern, wherein the first andsecond layer patterns belong to two different layers of an integratedcircuit (IC) design layout.
 2. The mask of claim 1, wherein the firstand second layer patterns are designed to be formed on respectivematerial layers on a semiconductor substrate; and the mask has threestates different from each other.
 3. The mask of claim 2, wherein themask substrate has a first transmittance to an exposure radiation of thelithography exposure process; the first mask material layer has a secondtransmittance less than the first transmittance; and the second maskmaterial layer has a third transmittance less than the secondtransmittance.
 4. The mask of claim 3, wherein the three states includea first state having the first transmittance, a second state having thesecond transmittance and a third state having the third transmittance.5. The mask of claim 4, wherein the first layer pattern is in the firststate; the second layer pattern is in the second state; and a field isin the third state.
 6. The mask of claim 3, wherein the thirdtransmittance is less than 6% of the first transmittance; and the secondtransmittance ranges between about 20% and about 80% of the firsttransmittance.
 7. The mask of claim 1, wherein the first layer patternincludes a first feature defined in a first opening of the first maskmaterial layer; and the second layer pattern includes a second featuredefined in a second opening of the second mask material layer.
 8. Themask of claim 7, wherein the second opening is aligned with the firstopening.
 9. The mask of claim 7, wherein the first feature is a viafeature; and the second feature is a metal line feature.
 10. The mask ofclaim 9, wherein the second feature is oriented in a first direction andspans a first dimension in a second direction perpendicular to the firstdirection; and the first feature spans a second dimension in the seconddirection, the second dimension being less than the first dimension. 11.The mask of claim 1, wherein the mask substrate includes fused quartz;the first mask material layer includes molybdenum silicon (MoSi); andthe second mask material layer includes chromium (Cr).
 12. The mask ofclaim 1, further comprising a third mask material layer disposed on thesecond mask material layer and includes MoSi.
 13. A mask for alithography exposure process, comprising: a mask substrate; a first maskmaterial layer over the mask substrate and patterned with a first layerpattern of an integrated circuit (IC); and a second mask material layerover the first mask material layer and patterned with a second layerpattern of the IC, wherein the first and second layer patterns aredifferent patterns of features of the IC.
 14. The mask of claim 13,wherein the mask substrate has a first transmittance to an exposureradiation of the lithography exposure process; the first mask materiallayer has a second transmittance less than the first transmittance; andthe second mask material layer has a third transmittance less than thesecond transmittance.
 15. The mask of claim 13, wherein the first maskmaterial layer includes a first feature; the second mask material layerincludes a second feature; and the first feature overlaps with thesecond feature and has a dimension not greater than the second featurefrom a top view.
 16. The mask of claim 13 further comprising a thirdmask material layer over the second mask material layer and patternedwith a third layer pattern of the IC.
 17. A mask for a lithographyexposure process, comprising: a mask substrate; a first mask materiallayer over the mask substrate and having a first feature correspondingto a first layer pattern of an integrated circuit (IC); and a secondmask material layer over the first mask material layer and having asecond feature corresponding to a second layer pattern of the IC,wherein: the first feature and the second feature have different biasesto an exposure radiation of the lithography exposure process, andwherein the first layer pattern and the second layer pattern aredifferent layers of an IC design layout.
 18. The mask of claim 17,wherein the mask is a transmissive mask.
 19. The mask of claim 17,wherein the mask is a reflective mask.
 20. The mask of claim 17, whereinthe first feature is a subset of the second feature from a top view.